dt-bindings: mmc: tmio_mmc: document Renesas R8A77970 bindings

Document the R-Car V3M (R8A77970) SoC in the R-Car SDHI bindings -- it's
the usual R-Car gen3 compatible controller with the internal DMA engine.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>